Touch sensing systems (“touch systems”) are in widespread use in a variety of applications. Typically, the touch systems are actuated by a touching object such as a finger or stylus, either in direct contact or through proximity (i.e. without contact) with a touch surface. Touch systems are for example used as touch pads of laptop computers, in control panels, and as overlays to displays on e.g. hand held devices, such as mobile telephones. A touch system that is overlaid on or integrated in a display is also denoted a “touch screen”. Many other applications are known in the art.
To an increasing extent, touch systems are designed to be able to detect two or more touches simultaneously, this capability often being referred to as “multi-touch” in the art. There are numerous known techniques for providing multi-touch sensitivity, e.g. by using cameras to capture light scattered off the point(s) of touch on a panel, or by incorporating resistive wire grids, capacitive sensors, strain gauges, etc into a panel.
US2004/0252091 discloses an alternative technique which is based on frustrated total internal reflection (FTIR). Light sheets are coupled into a panel to propagate inside the panel by total internal reflection. When an object comes into contact with a surface of the panel, two or more light sheets will be locally attenuated at the point of touch. Arrays of light sensors are located around the perimeter of the panel to detect the received light for each light sheet. A coarse reconstruction of the light field across the panel surface is then created by geometrically back-tracing and triangulating all attenuations observed in the received light. This is stated to result in data regarding the position and size of each contact area.
US2009/0153519 discloses a panel capable of conducting signals. A “tomograph” is positioned adjacent to the panel with signal flow ports arrayed around the border of the panel at discrete locations. Signals measured at the signal flow ports are arranged in a sinogram (b) and tomographically processed to generate a two-dimensional representation (x) of the conductivity on the panel, whereby touching objects on the panel surface can be detected. The presented technique for tomographic reconstruction is based on a linear model of the tomographic system, Ax=b. The system matrix A is calculated at factory, and its pseudo inverse A−1 is calculated using Truncated SVD algorithms and operated on a sinogram b of measured signals to yield the two-dimensional (2D) representation of the conductivity: x=A−1b. The suggested method is both demanding in the term of processing and lacks suppression of high frequency components, possibly leading to much noise in the 2D representation. US2009/0153519 also makes a general reference to Computer Tomography (CT). CT methods are well-known imaging methods which have been developed for medical purposes. CT methods employ digital geometry processing to reconstruct an image of the inside of an object based on a large series of projection measurements through the object.
One class of CT methods use Fourier transforms for image reconstruction, based on the so-called Projection-Slice Theorem, which stipulates that a 1D Fourier transform of projection values from a projection measurement results in a slice through a 2D Fourier transform of the image to be reconstructed. Thus, a method that operates a 1D Fourier transform on a sinogram of projection values will generate Fourier coefficients for data points arranged on radial lines in the Fourier domain, i.e. on a polar grid. The image may then be reconstructed by operating a 2D Fourier transform on the Fourier coefficients for the data points. To achieve appropriate computational speed and reconstruction accuracy, it may be desirable for the data points to be arranged on a Cartesian grid in the Fourier domain, e.g. to enable the use of inverse Fast Fourier Transforms (FFTs). Numerous techniques have been developed to transform the data points to a Cartesian grid, including interpolation techniques, e.g. as described in “The Mathematics of Computerized Tomography”, by F Natterer, 2001, in Chapter V. 2: “Fourier reconstruction”.
A further example of an interpolation technique is described in the article “Non-Equispaced Fast Fourier Transforms with Applications to Tomography” by K Fourmont, published in “Journal of Fourier Analysis and Applications”, Volume 9, Number 5, pages 431-450 (2003). This article proposes a 1D FFT, denoted 1D NER, that operates on equispaced data (the projection values) to generate a non-equispaced result (the data points in the Fourier domain). Specifically, each 1D NER is adapted to generate the data points at such locations along the radial lines in the Fourier domain, so as to allow the Fourier coefficients on the Cartesian grid to be generated by angular interpolation.
With respect to signal processing in touch systems, WO 2011/139213 discloses an improved technique for tomographic reconstruction based on signals from a touch system that operates by transmission of light inside a light transmissive panel. The signals, which represent detected energy on a plurality of detection lines across the touch surface, are processed to generate a set of matched samples, which are indicative of estimated detected energy for fictitious detection lines that have a location on the touch surface that matches a standard geometry for tomographic reconstruction. This technique enables the touch system to be designed with any arrangement of detection lines across the touch surface, while still allowing for the use of conventional tomographic reconstruction algorithms. These algorithms will generate an interaction pattern that represents the location of objects on the touch surface. With respect to existing Fourier-based reconstruction techniques, e.g. as exemplified above, the set of matched samples may form the sinogram (the projection values) that is processed by 1D Fourier transformation.
One challenge in respect of touch systems is that the interaction pattern may need to be generated in real time. This task is made even more demanding if the touch system is restricted in terms of processing speed or storage capacity, e.g. due to constraints imposed by a desire to reduce costs, limit power consumption, provide a certain form factor, etc.